1. Field of the Invention
This invention relates to a new and improved electron gun structure which uses high direct current negative voltage (HV) to form high energy electron beams for melting and casting of many different materials and for high vacuum evaporation sources for producing thin-film coatings on various substrates. More particularly, the invention relates to the use of a grounded metallic shield which functions as an electrode enclosing the filament leads and emitter structures of an e-Gun in a high vacuum chamber.
2. Description of Related Art
Vacuum furnaces conventionally employ e-Guns in the melting or casting of metals and other materials, employing HV as high as 50 KV and ingots as large as 10 tons. Vacuum furnaces may also be used in a high vacuum electron beam heated evaporation source in the manufacture of thin-film devices such as 1/4-wave stacks for the optics industry, or integrated circuits, and many other devices which use thin-film technology as part of the fabrication process. In this type of process, the HV value is as low as 4 KV and has a nominal value of 10 KV.
It has been found that in both types of e-Beam operations the operation can be disrupted by the buildup of an arc or glow discharge of the HV. This has an adverse effect in that the voltage can drop as low as a few hundred volts and the current flow rise to a level limited only by the ability of the external and PS circuits to provide the current.
The aforesaid arc or glow is caused by the fact that all conductive surfaces with negative HV impressed upon them in vacuum have a loss of a small number of electrons from such surfaces flowing outward to grounded surfaces, such as chamber walls and internal furnace equipment. Depending upon the pressure within the chamber, an electron may ionize many gas atoms at its energy threshold of about every 20 volts of its path from -10 KV to 0, or a potential tally of about 500 ions and a like number of electrons. In addition, the total current flow is further augmented by the flow of all the electrons from the ionized atoms and accompanying electrons from their part of the ion formation processes. This effect is known as a "Geiger Discharge" and is of a proportion whose current is limited only by the external circuits. The initial discharge current is augmented by the large numbers of secondary electrons liberated from the HV negative surfaces being bombarded by positive ions which are attracted to the negative voltage.
The foregoing has been a problem in the high voltage industry which has not, heretofore, been satisfactorily prevented.
One attempt at diminishing the start of arcing was performed by University of California physicists in about 1944 in connection with Calutron uranium isotope separation chambers. These attempts used insulators to hold down equipment parts comprising end cups with side skirts to reduce the amount of coating on the insulators. It was found that the amount of arcing around insulator ends diminished when the negative end was covered by a cap which reduced the voltage gradient impressed on the end surfaces. However, this phenomena is different from the present invention in that the negative end has HV impressed on it. The finite resistance of the insulator has a small flow of electrons down toward the positive end. A reduction in voltage gradient at the metal-ceramic interface of the insulator and the surrounding metal reduces arcing in that area but only in that area. Thus that development is contrary to the teaching of the present invention where applicant is covering HV surfaces with grounded structures.
As developments of e-Beam heated evaporators occurred around 1954, the solution accepted for reducing arc-downs was installation of a large size power resistor in the HV lead between the power supply and the e-Gun to limit the current flow to a low value where it rapidly self-quenched. Thus these attempts were aimed at limiting the current available to the arcs as a means of cessation and restoring power.
Steinmetz, in about 1915, had controlled the maximum current flow in a series of street arc lamps, and this was later adopted in a monocyclic resonant network to maintain a constant current flow with increased current demand resulting in HV dropping.
A faster and similar electronic current control device became available with the development of electronic power triodes and Tetrodes with the capability of several hundred KW capacity and voltage holding levels nearing 50 KV. The control function in this instance was merely that of monitoring the current flowing through the power supply and any increase, no matter how sudden, fed voltage into the tube grid circuit to shut off current flow and quench the arc. Reference is made to U.S. Pat. No. 3,204,096 disclosing apparatus for projecting an electron beam along a curved path having variable impedance. In this development HV changed if the current changed.
Other control methods have been used on the primary side of the HV transformer.
All of the foregoing methods were aimed solely at controlling and minimizing arc-down effect after such effect commenced.